Process for the remediation of contaminated soil

ABSTRACT

A process and apparatus for the remediation of soil is provided with a plug flow of soil and counter-current wash of water. The contaminant-containing soil with water is first treated to disengage the contaminant from the soil and form a liquid phase containing water and contaminant. The treated soil and liquid phase is continuously introduced into a wash zone at a first end of the wash zone, and the soil and the liquid phase are conveyed in successive and discrete portions through the wash zone between the first end of the wash zone and a second end of the wash zone to provide an essentially plug-flow of the soil through the wash zone. Wash water is introduced into the wash zone by continuously adding water to the discrete portions at the second end of the wash zone, and the wash water is conveyed through the wash zone between adjacent wash cells toward the first end counter to the plug flow conveyance of the soil and the liquid phase, such that contaminants in the liquid phase are removed from the liquid phase by the wash water. The soil and liquid phase with contaminants removed are continuously withdrawn from the wash zone as each discrete portion reaches the second end of the wash zone. The wash water containing removed contaminants is withdrawn from the discrete portions at the first end of the wash zone.

FIELD OF THE INVENTION

This invention relates to an apparatus and process for the remediationof contaminated soil.

BACKGROUND OF THE INVENTION

As a result of human activity there have been accidental, deliberate orunknowing releases of petroleum products, chemicals, metals andhazardous, toxic and radioactive substances (hereinafter referred to ascontaminants) into the environment. In many of these cases, thesereleases have been to the terrestrial environment. Upon contact withsoil, rock and other solids of the terrestrial environment, thesecontaminants mix and adsorb making recovery of the contaminantsextremely difficult. Left unrecovered these contaminants may betransported to the biosphere through surface or ground water, or throughdirect contact, resulting in a hazard to living orgaisms, includinghuman beings. There are increasing worldwide requirements to clean or"remediate" contaminated terrestrial solids by recovering theseundesirable contaminants and safely disposing of them.

Current technologies for remediating contaminated soil fall into twobroad categories: in-situ, those technologies which extract thecontaminant while the soil remain in-place; and ex-situ, thosetechnologies which excavate the contaminated soil and treat that soil bysurface process technologies. While in-situ technologies have provenuseful for certain types of contaminants such as volatile organics,in-situ technology has not proven to be generally applicable to a broadrange of undesirable contaminants.

Among the ex-situ technologies, there are two generic approaches:thermal and water-washing. Thermal processes rely on distillation,pyrolysis and/or combustion of the contaminants. Thermal processes areapplicable only to organic contaminants and are ineffective forremediation of soils with metal or radioactive contaminants. Thermalprocesses are inherently energy-intensive, resulting in high costs.Thermal processes also have air quality implications, requiring airquality permitting.

Washing of soil with water has been previously proposed, in for exampleU.S. Pat. Nos. 5,056,542, 4,951,417, 4,783,263, and "Soil WashingResults of EPA Tests for Effectiveness" The Hazardous Waste Consultant(May/June) 1989 pp 1-11 to 1-16. In general, proposed soil-washingprocesses rely on technologies largely developed in the extractivemetallurgy industry. Common to these approaches are a form ofpretreatment in which surfactant, caustic or other ingredients are addedto water and soil and the components are mixed in a stirred tank. Theresulting pre-treated mixture is then sent to an air flotation device inwhich the organic material is caused to float while theheavier-than-water ingredients such as minerals and soil, sink to thebottom and are removed.

Water washing technologies, using traditional extractive metallurgyconfigurations, are generally limited to petroleum hydrocarbons andother insoluble organics whose density is approximately equivalent to,or less than, that of water. These technologies are not by themselvesapplicable to water-soluble contaminants, which require additional soilwashing and water processing steps for removal.

Many of the difficulties with current soil-washing technologies relateto the requirements for a large pre-treatment vessel and multipleflotation stages. The stirred-tank process design inherently requiresmultiple stages, often with long residence times, to achieve ahigh-level of separation of the contaminants from the soil. Further, thecurrent soil washing processes separate primarily water-insolubleorganic contaminants. Therefore, any soluble mineral or soluble organiccontaminants remain with the water and are discharged, along with thesoil in the underflow of the flotation cells. A further process isaccordingly required to further decontaminate the underflow from theflotation stages.

A further problem of currently-practiced technology is the need forlarge volumes of water. Even though water is recycled, the overallvolume of water in the recycle loop may be several times the inventoryof the soil in the process. These high water-to-soil ratios, along withthe mixing of fresh water in the downstream settlers and thickeners,tend to dilute the recycle streams and make maintenance of process waterquality difficult. In addition, the high ratio of water to soil and theinherent inflexibility of multiple, stirred-tank reactor stages renderit difficult to control the process to achieve a high degree ofremediation. In addition, the contaminate in aqueous outflow from thecells is often highly diluted. For these reasons, the outflow oftenrequires a further cleaning and often requires large water processingfacilities.

OBJECTS OF THE INVENTION

It is, therefore, an object of the present invention to provide anapparatus and process for the remediation of contaminated soil thatachieves a high level of separation of contaminants from contaminatedsoil.

It is also an object of the present invention to provide an apparatusand process for the remediation of contaminated soil that usessignificantly less water than traditional washing techniques, with aminimum of make-up water and a minimum of water in the recycleinventory.

It is also an object of the present invention to provide an apparatusand process for the remediation of contaminated soil with a relativelyshort-residence time of the soil.

It is also an object of the present invention to provide an apparatusand process for the remediation of contaminated soil that is applicableto organic contaminants, whether water soluble or insoluble, and watersoluble inorganic contaminants, including salts, metals, andradionuclides.

Further objects of the invention will become evident in the descriptionbelow.

SUMMARY OF THE INVENTION

An embodiment of the invention is a process for the remediation of soilcontaining contaminant comprising;

(a) treating contaminant-containing soil with water to disengage thecontaminant from the soil and form a liquid phase containing water andcontaminant,

(b) continuously introducing the treated soil and the liquid phase fromstep (a) into a wash zone at a first end of the wash zone,

(c) conveying the soil and the liquid phase in successive and discreteportions through the wash zone between the first end of the wash zoneand a second end of the wash zone to provide an essentially plug-flow ofthe soil through the wash zone,

(d) introducing wash water into the wash zone by continuously addingwater to the discrete portions at the second end of the wash zone,

(e) conveying the wash water through the wash zone between adjacent washcells toward the first end counter to the plug flow conveyance of thesoil and the liquid phase, such that contaminants in the liquid phaseare removed from the liquid phase by the wash water,

(f) continuously withdrawing the soil and the liquid phase withcontaminants removed from the wash zone as each discrete portion reachesthe second end of the wash zone, and

(g) withdrawing wash water containing removed contaminants from thediscrete portions at the first end of the wash zone.

Another embodiment of the invention is an apparatus for remediation ofsoil containing contaminants comprising;

(a) a disengagement zone for treating contaminant-containing soil withwater to disengage the contaminant from the soil and form a liquid phasecontaining water and contaminant,

(b) a wash zone with a first end and a second end,

(c) a plurality of wash cells, each adapted to contain a discreteportion of soil and liquid phase,

(d) a conveyance means for successively conveying the wash cells fromthe first end to the second end,

(e) a soil feed means for introducing soil containing contaminants intothe wash cells at the first end,

(f) a soil output means for withdrawing the soil from the wash cells atthe second end,

(g) a water feed means for introducing wash water into the wash cells atthe second end,

(h) a wash water withdrawing means for withdrawing water from the washcells at the first end, and

(i) a water conveying means for conveying wash water from the second endto the first end by conveying water between the wash cells in adirection counter to the conveyance of the wash cells.

By "soil," as used herein, is meant consolidated and unconsolidatedmineral substances, and includes, but is not limited to naturallydeposited materials that cover the surface of the earth, ground mineralmaterials resulting from mining, ores, excavation, and the like, clays,gravels, sand, silt, and fill materials, and may or may not also containorganic and humic matter.

Disengagement of the contaminant from the soil matrix is required toremove bound contaminant from the solid soil phase and create a liquidphase containing contaminant. The contaminated soil is treated withwater, along with any required chemical additives to form the liquidphase. As use herein, "liquid phase" means a homogeneous ornonhomogeneous liquid aqueous phase that contains contaminant. Theliquid phase may be in any liquid form wherein contaminant is carriedwith the liquid, e.g.; water with contaminate dissolved therein; waterwith suspended or emulsified contaminate, either as a solid or liquid;and water with liquid or solid contaminant floating as a separate phaseon the water. In the process of the invention, the liquid phase isconveyed along with the soil, and contaminants that have been disengagedfrom the soil are extracted from the liquid phase. A product of thepresent process is a mixture of soil and extracted liquid phase, whichmay be separated into a decontaminated soil stream and an extractedliquid phase stream. The extracted liquid phase may then be disposed ofby conventional methods, or recycled into the process, e.g., as feed forthe disengagement means, or into the feed for the wash water.

The means for disengagement may be any suitable means for freeing thecontaminant from the soil matrix to form the liquid phase. Preferably, aplug flow disengagement means, as described in the example below isused. However, any plug flow, or mixed reactor system is suitable. Thespecific disengagement process will depend on the nature of thecontaminant. For water soluble contaminants, an environment to dissolvethe contaminant from the soil into water is required. Certain solidcontaminants may be freed from the soil by appropriate floatationreagents that form a solid-containing foam on the surface of the liquidphase. Insoluble liquid contaminants, such as petroleum and itsderivatives, can be treated with aqueous surfactant solution to form aliquid phase with the contaminant either suspended or floating on thetop.

The wash zone is the zone wherein contaminant is extracted by wash waterfrom the liquid phase, and is between the soil-inlet or first end andthe soil-outlet or second end. In the preferred embodiment, the washzone is preferably provided by a cylindrical drum with its axis on atilted or skewed alignment, i.e., on neither a vertical nor a horizontalaxis. Within the drum is a helical weir wall, which functions as themeans for conveying the soil and liquid phase through the wash zone. Thehelical weir wall acts as a screw conveyor that moves the soil andliquid phase as discrete portions from the first end of the drum to thesecond end of the drum. Since the weir wall conveyor is in the form ahelix and the drum is tilted, the soil and liquid phase can be conveyedas discrete portions in separate wash cells along the cylinder. The drumand screw helix may extend beyond the first and second ends of the washzone (the region between the wash water feed and the wash waterwithdrawing mean) to provide mixing or disengagement. For example, thephysical soil inlet of the drum may be one or more cells below the washwater feed to insure mixing of the soil in the cells, and to providefurther disengagement of the contaminant.

The portions of the helical wall between the wash cells, those thatcontain the discrete portions of the soil, also function as weir forwash water flowing between adjacent cells. Wash water is introduced intothe soil-outlet or second end, and, because of the tilted alignment ofthe cylinder, wash water flows from the second end down through thecylinder, flowing from cell to cell over the weir wall, to thesoil-inlet or first end. The weir wall essentially prevents a back-flowof water to an adjacent cell in the direction toward the second end,providing an continuous flow of water in the other direction, orcounter-current to the plug-flow of the discrete soil and liquid phaseportions. This configuration provides for staged dilution ofcontaminated water without back-mixing.

Although, the means for conveying the soil through the wash zone ispreferably provided by a helical weir wall on the interior wall of acylinder that functions as a screw conveyer, as described above, anymeans that conveys soil through the wash zone in discrete portions,while allowing a counter current flow of water as described below, issuitable. For example, an inclined chain or belt conveyor can move soilcarrying buckets or wash cells upwards along the conveyor, while wateris cascaded down the conveyer from bucket to bucket. Alternately, arotating auger conveyor can be used to convey soil in discrete portionsup a tube. Ports in the auger permit water to flow down through augerassembly in a counter-current manner from soil portion to soil portion.

The apparatus and process of the invention has been devised to utilizethe kinetic efficiency of a plug-flow process design. In plug flowdesign, partially cleaned liquid phase and soil do not mix with thefully contaminated soil and liquid phase entering the process,permitting the contamination to be extracted to low levels. Thiscontrasts with the prior-art mixed reactor designs, wherein partiallyextracted materials back-flow and are fully mixed with fullycontaminated feed as it enters the process. The contaminant extractionin such a mixed reactor system, therefore, is limited to theconcentration in the mixed system. In addition, the present processincorporates a counter-current wash water flow. Therefore, the cleanerwash water is used to treat the wash cells with the lower contaminantconcentration, which permits reduction of the contaminants in the soilto low concentrations. In addition, the plug-flow and counter-currentextraction regime permits a relatively low flow of water to achieve anefficient removal of contaminants.

In a preferred embodiment, contaminated soil, is disengaged by additionof water and chemicals, and the soil with contaminate-containing liquidphase is charged to a rotating drum in which the soil is conveyed fromthe first end of the drum to the second end by means of a helical orscrew conveyor fixed to the interior of the drum. Fresh wash water isadded to the soil-outlet or second end of the drum, which has beenslightly elevated such that the wash water cascades downward in acounter-current direction in relation to the motion of the soil. Thewalls of the screw conveyor act as water weirs, preventing anyback-mixing in the axial direction of the flow of water. Between theweir-like walls of the screw conveyer, soil and liquid phase is conveyedin cells as discrete portions, with little soil mixing between the cellsand no fluid back-mixing toward the soil-outlet or second end, providingan essentially plug-flow regime of the soil and liquid phase.

Extracted liquid phase with soil is discharged from the elevated secondend of the drum and wash water which contains the original contaminantsis discharged from the lower first end of the drum. In a preferredembodiment, wash water is then physically separated from any insolublecontaminant and recycled to the system to conserve the water and processadditives.

The process of the invention is applicable for the removal of anysubstance or contaminant that can be washed from the soil by water,either alone or with added surfactant, pH modifiers, solubilizers, orother additives in the water. Such contaminants include materials thatare solubilized or disengaged from the soil matrix by an aqueous system.Examples of contaminants include light, heavy, and viscous hydrocarbons,soluble and insoluble organic chemicals and soluble metals and minerals,including radionuclides. The process of the present invention is alsoapplicable to removal of naturally occurring substances, such as metals,minerals, and the like. For example, the extraction from ores of goldusing cyanide solutions, and the extraction of metal oxides using acidicsolutions. Accordingly, the term contaminant, as used herein, includesnaturally occurring substances in the soil and substances occurring dueto human activity.

The improved results that can be achieved by practice of the inventioncan be illustrated by a brief theoretical description of the invention.Remediation of contaminated soil requires two physiochemical steps. Thefirst step is to cause the contaminant to disengage from its physical orchemical association with the soil. The second step is to physicallyseparate the disengaged contaminant from the soil particles. Processeffectiveness may be defined as the degree to which the objectives ofthese two process steps are achieved. Process efficiency is defined asthe approach to maximum theoretical effectiveness.

The Disengagement Step

Disengagement involves the release of the contaminant from the soil sothat it can be incorporated in the liquid phase comprising water andcontaminant. For contaminants that are very soluble in water and/or arelightly bound to the soil, disengagement may occur quickly in the washcells near the first end of the wash zone. However, for mostcontaminated soils, the soil must usually be treated with water andchemicals in a separate disengagement means to disengage the contaminantfrom the soil. For water-insoluble contaminants, the treatment mayinclude surfactant, pH modifiers, or other materials, such that thecontaminant with water forms a two phase liquid, or an aqueous phasewith suspended solid or liquid contaminants. For water-solublecontaminants, the pretreatment is under conditions to permit thecontaminant to dissolve in water. After the disengagement treatment, thesoil with the liquid phase, i.e., the treatment water containingcontaminant, is introduced into the wash zone at the first end aspreviously described.

In a multicomponent, liquid-solid system, the partitioning of componentsbetween the liquid and solid state is governed by equilibriumthermodynamics. By definition, at equilibrium, the free energy of acomponent or contaminant in the water phase (determined by thecontaminant level in the liquid phase) is equal to its free energy in(or on) the soil or solid phase. In a system in which it is desirable toeffect a disengagement of contaminant (the components), from the surfaceof soil (the solid), efficiency may be defined as the approach toequilibrium. This is illustrated in Equation 1 for any component, A.##EQU1## Where [A]_(l) and [A]_(s) are the levels of contaminant in theliquid phase and solid phase, respectively, which are achieved in aparticular disengagement process, [Ao]_(l) and [Ao]_(s) are the likecontaminant levels at equilibrium, and E_(DIS) is the theoreticaldisengagement efficiency.

In practice, it is desirable to maximize the partitioning of contaminantinto the water phase, or find conditions where:

    [Ao].sub.1 /[Ao].sub.s >>1                                 (2)

The objective of maximizing this partitioning to the water phase isachieved through adjustment of water chemistry and temperature.

It is also desirable to maximize the rate at which equilibrium isapproached. This objective is achieved through adjustment ofmechanical/physical processes. In addition, equilibrium is approachedmore easily in a cocurrent, plug-flow system, which is the preferredregime for disengagement in the present invention. A cocurrent,plug-flow system provides approximately a first order kinetic regime.For such a co-current, first-order, plug-flow system, the rate at whichequilibrium is approached may be modeled through the use of theempirical, first-order, kinetic expression:

    E.sub.DIS =1-ae.sup.-bt                                    ( 3)

where a, b are constants, which are particular to each system, and t isresidence time. In the practice of the invention, the residence time (t)should preferably be sufficient to achieve E_(DIS) >0.90.

The Wash Step

The objective of the wash step is to physically separate contaminantswhich are disengaged and are with the liquid phase from the solid, orsoil phase. Contaminants can be with the liquid phase as a component insolution or as a nonsoluble suspended liquid or solid or a separatephase from water in a multiphase liquid. Separation of thesecontaminants usually utilizes separation based on density differences.Separation of contaminants soluble in the water phase requires exchangeof the contaminated water with uncontaminated or less contaminatedwater. In the apparatus and process of this invention, this isaccomplished by the counter-current wash with staged dilution aspreviously described. Insoluble, lighter-than-water contaminants arewashed toward the soil-inlet or first end at approximately the rate ofwater flow over the weirs, but in no case at less than the rate of washachieved for soluble contaminants.

Process effectiveness (R_(A)) may be measured in terms of the ratio ofthe level of contaminant remaining with the soil after washing (A_(f))to that originally contained on the soil (A_(o)). ##EQU2## The processor washing efficiency (E_(WASH)) is defined as the calculated ortheoretical minimum process effectiveness ratio divided by the observedor actual ratio. ##EQU3## The efficiency of an individual cell(E_(CELL)) is defined as the change in concentration about the celldivided by the theoretical maximum change in concentration possible forthe cell: ##EQU4## The effectiveness of the overall process dependslargely on the efficiency of the cells, the number of cells or stages,and the counter-current or internal flow ratio (R_(W)), i.e., the ratioof the amount of wash water flowing counter-current from cell to cellcounter to the soil movement, to the amount of liquid phase carriedalong with the cells. For purposes of illustration, the cell efficiencyE_(CELL) can be assumed to be ideal, i.e., equal to 1 (and therefore theprocess efficiency E_(WASH) =1). The theoretical process effectiveness(R_(A)) may then be calculated as a function of the number of cells orstages and the counter-current flow ratio. This data is shown in Table Afor a variety of internal flow ratios (R_(W)) and number of cells (N).The data in Table 1 illustrates the wide range of flexibility andcontrol over the level of decontamination which can be achieved in thepreferred apparatus.

                  TABLE A                                                         ______________________________________                                        Ideal Process Effectiveness (R.sub.A) for E.sub.CELL = 1                      R.sub.W                                                                            5       10      12    14    16    18    20                               ______________________________________                                        0.5  0.51    0.50    0.50  0.50  0.50  0.50  0.50                             0.8  0.27    0.22    0.21  0.21  0.20  0.20  0.20                             1.0  0.17    0.091   0.077 0.067 0.059 0.053 0.048                            1.2  0.10    0.031   0.021 0.014 9.4E-3                                                                              6.5E-3                                                                              4.4E-3                           1.5  0.048   5.9E-3  2.6E-3                                                                              1.1E-3                                                                              5.1E-4                                                                              2.3E-4                                                                              1.0E-4                           2.0  0.016   4.9E-4  1.2E-4                                                                              3.1E-5                                                                              7.6E-6                                                                              1.9E-6                                                                              4.8E-7                           3.0  2.8E-3  1.1E-5  1.3E-6                                                                              1.4E-7                                                                              1.5E-8                                                                              1.7E-9                                                                              1.9E-10                          4.0  7.3E-4  7.2E-7  4.5E-8                                                                              2.8E-9                                                                              1.7E-10                                                                             1.1E-11                                                                             6.8E-13                          5.0  2.6E-4  8.2E-8  3.3E-9                                                                              1.3E-10                                                                             5.2E-12                                                                             2.1E-13                                                                             8.4E-15                          ______________________________________                                    

From Table A it can be seen how process effectiveness (R_(A)) can beincreased by increasing the internal flow ratio (R_(W)) and/or thenumber of cells in the process (N). In addition, it can also be seen howextremely low amounts of contaminants in or on the soil, as shown by theprocess effectiveness (R_(A)) can be achieved. In an actual process,process efficiency (E_(WASH)) is expected to be less than 1 (the idealcase), but results of an actual process would follow a similar pattern.To achieve the same low contamination levels of Table A where E_(CELL)<1, the number of cell stages (N) and/or the internal flow ratio (R_(W))can be increased. In addition, the cell efficiency may approach 1 if theresidence time of each cell in the process is long enough.

In summary, by practice of the invention a high degree of flexibility inachieving desired results is possible. By manipulating processvariables, such as the internal flow ratio (R_(W)), a desired degree ofremediation can be achieved. This contrasts with the prior-art stirredtank designs which have an inherent limit to the degree of remediation,and do not have the degree of flexibility for achieving a desired resultas the present invention.

In the present invention with the plug-flow for the soil and with acounter-current flow of wash water an unexpectedly high degree ofdecontamination is achieved. From a practical standpoint, such resultsare not achievable using traditional stirred reactor washing processes.To achieve such results in a stirred tank washing process, an extremelylarge amount of wash water, and numerous washing stages would berequired, which from a capital cost and operating cost standpoint wouldnot be practical. The preferred cocurrent plug flow design of thedisengagement step and the counter-current plug-flow design of the washstep are designed to minimize residence time and therefore, increasethroughput capacity for a given reactor space. This results in lowercapital and operating costs.

The process of the invention requires low energy and utilities, reducingdemand on valuable resources. In addition, since the present process isessentially a water based washing process, there are essentially nohazardous emissions or effluents to the atmosphere. The final waterwaste stream containing the contaminants has a relatively highcontaminant concentration in small volume, which allows the waste streamto be easily treated and disposed of.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow diagram showing a remediation process incorporating anapparatus of the invention.

FIG. 2 is a cross-section of a preferred apparatus of the invention.

FIGS. 2a to 2c are details of the cell lifters from the apparatus ofFIG. 2.

FIG. 3 is a cross-section of another preferred apparatus of theinvention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a flow sheet showing a simplified process configuration wherethe apparatus of the invention can be used. Contaminated soil enteringon flow line 101 is first screened of oversized material at screen 103and sent to a disengagement apparatus 105 via line 104 where it is mixedwith water (entering from line 107), along with any other chemicals suchas pH modifiers, and surfactants, to enhance the disengagement of thecontaminant from the soil. Soil, and liquid phase (water with disengagedcontaminant) leaves through line 109 and is charged to a wash apparatusof the invention 111, where it is subjected to counter-current washunder plug flow. Clean wash water enters through line 113. Cleaned soilis discharged from wash apparatus 111 through line 115 and dewatered(separated from the extracted liquid phase) in separator 117, andremoved from the process though line 119. The separated water isrecycled via line 121 to the wash apparatus 111 as a supplement to thefresh water feed from line 113.

The wash water containing contaminant, and fine soil particles, isdischarged from the wash apparatus 111 via line 123 to settler 125 andallowed to settle. Floating contaminants in the wash water are sent vialine 127 to a product treatment facility (not shown), utilizingmore-or-less conventional petroleum and/or chemical processes. Waterfrom the settler 125 is recycled via line 129 to the disengagementdevice 105 for conservation of water and chemicals. Make-up chemicalsare added through line 130 as required.

The wet fines are sent from the settler 125 via line 131 to a separationapparatus 133. Conventional water separation solids technology may beused here, including, but not limited to, thickening, coagulation,filtration, inclined-plate settling, centrifugation, etc. Waterrecovered from this unit via line 135 may also be recycled or sent totreatment facilities prior to discharge or recycle. The fines from theseparation device are recovered through line 137.

Referring to FIG. 2, which is a simplified cross-section of a washapparatus of the invention, apparatus comprises a drum 201 with the axisof rotation 225 on a tilted alignment (at angle 227). The diameter 203and length 202 of the drum depend upon such factors as desired capacityand height, width and weight transport restrictions. The drum 201 isinternally outfitted with a helical weir wall 205, which has a height204 and pitch 206 determined by the diameter 203 of the drum 201, thedesired residence time, the speed of rotation of the drum and factorspertaining to mixing energy. A wash cell 207 for transporting a soilportion through the drum is formed by a single revolution of the helicalweir wall 205. In the drum 201 a wash zone 209 is provided, with asoil-inlet or first end 211 and a soil-outlet or second end 213. At thesecond end 213 is a wash water feed 215 for introducing water in washcells 207 as they pass to the second end 213. As wash cells 207 areconveyed through the wash zone 209, successive wash cells 207 reach thesecond end 213 in turn, and receive water from the water feed 215. Atthe first end 211 is a line 217 for withdrawing water which has cascadedover the weir walls. The first end 211 and second end 213 of the washzone may correspond with or are near the physical ends of the drum 201.However, if it is desired to convey wash cells 207 containing thediscrete soil portions through a drum portion before entering or afterleaving the wash zone 209 for further mixing or processing, the drum 201ends may extend any distance beyond the first end and second ends 211,213 of wash zone 209.

FIGS. 2a, 2b, and 2c, are detail views of a wash cell 207, from theside, the end, and the top respectively. In order to provide a mixing inthe cells 207, lifters 219 of a specified height profile, extend acrossthe segments between the rotations of helical weir. The height profileand the number of lifters 219 are designed to provide adequate solutionmixing energy while, at the same time, allow fine soil particles tosettle and be conveyed with the soil. The lifters 219 are tiltedbackward at an angle 221 to provide convenient draining as the lifter219 passes from a submerged state to a non-submerged state. The lifters219 may also be mounted at an angle 223 to the drum axis 225 (in FIG. 2)to provide thrust of the coarsest material in the direction of soilflow.

The axis 225 of the drum 201 is tilted at an angle 227 to the horizontalto provide counter-current washing by means of gravity. The angle 227 ischosen so as to prevent back mixing of water as it passes over the screwconveyor weir walls 205.

During operation of the apparatus of FIG. 2, contaminated soil entersvia line 229 at the first end 211 of the drum and enters a cell 207. Asthe drum 201 rotates on axis 225, the helical arrangement of the weirwall 205 causes the cell 207 and its contents to migrate towards thesecond end 213. The soil and liquid phase are conveyed a discreteportion within the cell 207, without any substantial mixing withadjacent cells 207. At the second end 213, cleaned soil is ejected bythe rotation of the helical weir wall 205 and is dumped via line 231into a suitable filtering or dewatering apparatus 233, which is here notspecified and shown as a box. Water from the dewatering apparatus 233,is recycled via line 235 by introducing it into the second end 213. Thewater from the recycle 235 and fresh water from the wash water feedintroduced through line 215 are introduced into a cell 207 at the secondend 213. The wash water cascades from that cell to adjacent cells 207successively in turn toward the first end 211, which is counter to theplug-flow of the soil. The weir wall 205 that defines the discrete cells207 also prevents the backflow of water between cells 207 toward thesecond end 213. The wash cells 207 contain the liquid phase, i.e., watercontaining contaminant disengaged from the soil, and is carried alongwith the discrete portions of soil. The liquid phase containing thedisengaged contaminants is successively diluted and the concentration ofcontaminant remaining with the water discharged with the soil at thesecond end 213 is lowered to a concentration in accordance to theoperating conditions, as illustrated in Table A. Thus, the system isessentially a plug-flow cleaning system, with wash water flowingcounter-currently to the liquid phase conveyed in the wash cells.

The process effectiveness (R_(A)), the ratio of contaminants in thewater leaving the last cell to the separator 233 to the contaminants inthe waste water stream, can be easily controlled, primarily byregulating the internal water flow ratio, the ratio of the rate of flowof the liquid phase up through the drum to the rate of thecounter-current flow of water down through the drum. Other factorscontrolling process effectiveness, are the number of cells in the washzone, as described above. The process effectiveness may be limited tothe extent that the soil may contain contaminants in spaces accessibleonly through small pores, through which the washing medium is unable topenetrate.

In FIG. 3 is shown a preferred apparatus of the invention incorporatingan integral disengagement unit within the wash drum. The drum 201 issimilar to that of FIG. 2, and is shown with the same reference numbers.

The disengagement unit 301 is preferably configured with a helical orscrew conveyor 303 fixed to the inside of a cylindrical tube or drum305. The screw conveyor 303 provides a plug flow through the drum 305.In certain installations, a partial plug flow may be achieved withoutthe screw conveyor, i.e., with no extra conveyor means inside the drum305. Soil and water are introduced at the elevated or feed end 307, andaqueous phase and disengaged soil are withdrawn from the disengagementzone outlet 309 and deposited at the first end 211 of the wash zone 209.Lifters (not shown) may be attached to the outer edges of the helix, toprovide mixing, stirring, shear stress and other visco-mechanical forcesdesigned to accelerate the rate of approach to equilibrium. The generalkinetic equation defining this process is given in Equation 3 above. Thedimensions of the pitch and diameter of the helix are determined by therequired residence time and the desired throughput rate. Thedisengagement unit may be fixed to the rotating wash apparatus androtated at the same angular velocity for ease of operation. For such aconfiguration, the left or right handedness of the disengager helix mustbe opposite that of the helical weir wall of the wash apparatus. Thedisengager apparatus functions as a co-current plug flow reactor and theapproach to equilibrium follows first order kinetics as described above.Alternately, the disengagement unit may not be fixed and may rotateindependently, which would provide more flexibility of operation of thedisengager apparatus, but at a cost of more mechanical complexity. Thedisengager apparatus may also be a totally separate unit in which soilis conveyed between the disengager zone and the wash zone by a suitableconveyer means, such as a conduit pipe or a conveyor belt.

EXAMPLES Example 1A Test of Wash Efficiency for Water-SolubleContaminant Using Low Water Rate

A wash drum and disengager essentially as illustrated in FIGS. 2 and 3,was used. The wash drum was outfitted with ten cells in thecounter-current wash zone was charged continuously with soil containinga water-soluble contaminant. A test sample of soil was prepared bymixing with a water soluble material (#40 Red Food Coloring), which wasused as a simulated water soluble contaminant for removal from the soil.The disengagement step was carried out with an apparatus essentially asFIG. 3, above, and operated as described for FIG. 3.

For the wash step, fresh wash water was added at a rate of 2 gal/min.The water discharged from the second end with the liquid phase wasrecycled at a flow rate of 10 gal/min into the wash water. Wash waterdischarged from the first end was recycled at a flow rate of 6 gal/mininto the soil feed, and became incorporated into the liquid phase. About2 gal/min of wash water discharged from the first end was withdrawn as adrag or water output stream. The internal flow ratio (R_(W)) was 1.2.

Under these conditions, the theoretical process effectiveness (R_(A))was 0.115. The ratio accounts for the fact that at steady state thewater recycled from the second end contained small concentrations ofcontaminant, as compared with the data in Table A where fresh water wasassumed for the counter-current wash. The actual measured processeffectiveness ratio (R_(A)) was 0.117, demonstrating a processefficiency (E_(WASH)) of 0.98.

Example 1B Test of Wash Efficiency for Water-Soluble Contaminant at HighWater-Flow Rates

A second experiment was conducted under conditions as described inExamples 1A, above. In this case, the internal flow ratio (R_(W)) was1.5. Under these conditions, the theoretical process effectiveness(R_(A)) was 0.00977. The experimental process effectiveness (R_(A)) wasequal to 0.0114, demonstrating a wash efficiency (E_(WASH)) of 0.86. Theprocess effectiveness ratio (R_(A)) was about one-tenth of that ofExample 1A, which indicates the absolute effectiveness for soilremediation was about ten times that of Example 1A. This relationship isconsistent with the relationship of the process effectiveness shown inTable A above.

Example 2 Test for Removal of Petroleum Hydrocarbons from Soil

An apparatus of the invention as illustrated in FIG. 3 was constructedwith a continuous soil feed mechanism.

The wash drum was 35 inches in diameter and the helical weir wall had apitch of 6 inches. The drum was about 10 feet long providing 20 washcells in the wash zone. The disengagement apparatus was fixed within thewash drum, and placed to eject soil and liquid phase at the first end ofthe wash drum. The diameter and length of the disengagement drum were 16inches and 12 feet respectively, and it had an internal screw conveyorwith a pitch of 16 inches. The wash drum and disengagement drum wererotated about 4 rpm on an axis tilted 5° from the horizontal. Thisprovided about 2 minutes for the soil residence time in the disengager,and about 2.5 minutes residence time in the wash drum.

Soil contaminated with 2.5 wt. % motor oil was charged to the system ata rate of two tons/hour. The internal flow ratio (R_(W)) was 1.5. Thewater pH was maintained between 8 and 10. The system was operated withwater recycle as described in Example 1 for one hour to achieve a steadystate, after which the results in Table B were obtained.

                  TABLE B                                                         ______________________________________                                        Motor Oil Contaminated Soil Remediation                                       ______________________________________                                        Contamination on Soil Feed                                                                             25000 ppm                                            Contamination on Discharged Soil                                                                        2000 ppm                                            Contamination in Water Drag Stream                                                                       50 ppm                                             Contamination on Thickener Bottoms                                                                      600 ppm                                             ______________________________________                                    

The remaining contaminant was recovered from the wash output as floatingoil using skimming devices. The wash water drawn from the process wastreated by a thickener to remove suspended fines.

While this invention has been described with reference to certainspecific embodiments and examples, it will be recognized by thoseskilled in the art that many variations are possible without departingfrom the scope and spirit of this invention, and that the invention, asdescribed by the claims, is intended to cover all changes andmodifications of the invention which do not depart from the spirit ofthe invention.

What is claimed is:
 1. A process for remediation of soil containingcontaminants comprising;(a) treating contaminant-containing soil withwater to disengage contaminant from the soil and form a liquid phasecontaining water and contaminant, (b) continuously introducing thetreated soil and the liquid phase from step (a) into a wash zone at afirst end of the wash zone, (c) conveying the soil through the wash zoneby moving the soil and the liquid phase between the first end and asecond end of the wash zone in successive and discrete portions withessentially no mixing of the soil between the discrete portions andessentially no flow of water between discrete portions in the directionfrom first end toward the second end, (d) introducing wash water intothe wash zone by adding the wash water to each discrete portion as saiddiscrete portion reaches the second end of the wash zone, (e) conveyingby gravity the wash water through the wash zone from the second endtoward the first end counter to the conveyance of the discrete portions,such that the wash water passes between and through the discreteportions to remove contaminants from the liquid phase in each discreteportion, (f) withdrawing each discrete portion from the wash zone assaid discrete portion reaches the second end of the wash zone, saiddiscrete portions reaching the second end containing soil and liquidphase with contaminant removed, and (g) withdrawingcontaminant-containing wash water from each discrete portion as saiddiscrete portion enters the wash zone at the first end.
 2. The processof claim 1 wherein the water in step (a) contains additives to assistdisengagement of the contaminants from the soil.
 3. The process of claim1 wherein the withdrawn soil and liquid phase from step (f) is treatedto separate liquid phase from the soil.
 4. The process of claim 3wherein at least a portion of the separated liquid phase is recycled tothe wash water introduced into the wash zone in step (d).
 5. The processof claim 3 wherein at least a portion of the separated liquid phase isrecycled to the water in step (a) to disengage the contaminant from thesoil.
 6. The process of claim 1 wherein at least a portion of the washwater withdrawn in step (g) is recycled to the water in step (a) todisengage the contaminant from the soil.
 7. The process of claim 1wherein the wash water withdrawn in step (g) is treated to removecontaminants in the wash water.
 8. The process of claim 1 wherein thewash water withdrawn in step (g) is treated to remove fine solidmaterials from the soil.
 9. The process of claim 1 wherein the internalflow ratio is greater than
 1. 10. The process of claim 1 wherein theamount of contaminant in the wash water withdrawn in step (g) iscontrolled by varying the number of discrete portions being conveyedwithin the wash zone.
 11. The process of claim 1 wherein the amount ofcontaminant in the wash water withdrawn in step (g) is controlled byvarying the internal flow ratio.